-12 is simply the number -12. It is a negative number. It is located 12 units to the left of zero on the number line (where right of zero is positive, and left of zero is negative).
|-12| is "absolute value of -12"
The absolute value of a number is the distance the number is from zero on the number line. A distance can be zero or a positive number, but never a negative number. The distance from the number -12 to 0 on the number line is 12 units. The absolute value of -12 is 12. The absolute value of a number is non-negative. That means it can be zero or positive, but it cannot be negative.
